This study is a cooperative, randomized clinical trial to determine the safety and efficacy of phototherapy for treatment of neonatal hyperbilirubinemia by comparing treated with untreated infants under specific conditions. Babies were randomized by weight (less than 200 200-2449, and greater than 2,500 grams) to phototherapy or non-phototherapy. Infants 2,000 grams and above were admitted to the study when their bilirubin reached levels specified in the study protocol whereas all infants under 2,000 grams were admitted. Physical, neurological and mental development of these infants will be followed through six years of age.